In the magnetic picture, the gauge-field production parameter \(\xi \)  thus receives a correction that depends on the strength of the induced fermion current.
This observation is the starting point for our first estimate of the efficiency of gauge-field production, which has been proposed for the first time in Ref. [1]}.
Besides, there is also the electric picture, developed in Refs. [2]}, [3]} and discussed in more detail below, in which the fermion current is considered to be primarily controlled by the electric field, \(\mathbf {J} = J/E \mathbf {E}\) .
Consequently, instead of a correction to the instability parameter \(\xi \) , the fermion current induces a finite generalized conductivity.
